Film curl is of critical importance in the handling and processing of photographic films. Because of the high humidity sensitivity of emulsion layers and the large differences in humidity-expansion coefficient (HEC) among the various layers in a typical photographic film, the curvature of the film is particularly sensitive to variations in relative humidity (RH). This problem is especially acute in films wherein a polyester such as polyethylene terephthalate is used as the film base because such films have a very low humidity-expansion coefficient, and it becomes more severe for thinner films.
It is desired to make photographic elements thinner in order to enable more pictures to be taken on a film housed in cartridges currently utilized or to provide a film cartridge smaller in size to achieve the number of exposures equal to that presently available. This reduction in the thickness of the photographic elements would in turn permit the manufacture of smaller cameras. While cellulose triacetate film base has been for a long time the primary material of choice, it does not have the physical strength characteristics necessary in order to reduce the thickness of the support upon which photographic emulsion layers are applied. Polyethylene terephthalate, on the other hand, while it has the necessary mechanical characteristics suffers from problems with respect to curl at various relative humidity conditions.
One technique known in the art to control curl is to apply a pelloid (gelatin) layer to the side of the film base opposite to the side the photographic emulsion is applied. This is disadvantageous because the pelloid layer can not be applied during the manufacture of the film base, thus requiring a separate coating operation which greatly increases the capital and operating costs.